In general, an acoustic transducer converts a driving force into acoustic radiation. Typically, a driving means is supplied with energy and vibrates in response thereto. The vibrating driving means is coupled to a flexural surface which projects the vibrations as acoustic radiation. Accordingly it is important to efficiently couple the driving means with the flexural surface
By way of illustrative example, coupling will be described as it pertains to a flextensional transducer. As shown in FIG. 1, a typical flextensional transducer 10 projects acoustic radiation into the ocean using a vibrating shell 12. The device that provides the driving force to vibrating shell 12 is typically an electrically actuated piezoelectric or magnetostrictive ceramic stack driver 14. Stack driver 14 is situated along major axis 100 of transducer 10. The geometry of transducer 10 is designed such that the vibrating shell's resultant displacement across its minor axis 200 is much greater than the displacement of stack driver 14. This amplification, in effect attributable to the shell geometry, is an efficient means of converting the stack's driving power into acoustic radiation. Between each end of stack driver 14 and shell 12 is a coupler component which is typically machined from aluminum in the shape of a "D" and is therefore referred to as a D-insert. D-insert 16 transfers the driving force/displacement of stack driver 14 to shell 12.
In FIGS. 2(a) and 2(b), a side view of one end of transducer 10 is shown with shell 12 in its undeformed state and deformed state, respectively. The geometry of D-insert 16 is such that its radius of curvature is less than the end radius of the inner surface 12i when shell 12 is in its undeformed state. This results in an area of contact between inner surface 12i and D-insert 16 as indicated by contact arrows 18. As shell 12 deforms during deflection, its end radius decreases so that the area of contact between inner surface 12i and D-insert 16 increases as indicated by contact arrows 18 in FIG. 2(b). Note that this also results in an increase in transverse structural support by D-insert 16 to shell 12 as indicated by transverse force arrows 20.
As shell 12 continues to deform to its greatest or worst case deflection, the end radius of inner surface 12i can potentially decrease to where it is less than that of the radius of D-insert 16. This results in an adverse condition of a gap 22 forming between inner surface 12i and D-insert 16 as shown in FIG. 3. This condition, known as "gapping", sets up an unfavorable coupling condition because shell 12 is only in contact with D-insert 16 at points 24. As a result, shell 12 may pivot about contact points 24 independent of deflection generated by stack driver 14. Thus, coupling efficiency is greatly reduced and may degrade to the point where shell 12 can deflect out-of-phase with the deflections generated by stack driver 14. In addition, owing to the reduced contact area between inner surface 12i and D-insert 16 caused by gapping, localized stresses in shell 12 around contact points 24 increase severely.
One solution to gapping is to design D-insert 16 with a radius of curvature that will always be smaller than the worst case deformed state of shell 12. However, while solving the gapping problem, a smaller D-insert offers reduced transverse structural support for shell 12 since the available contact area between the D-insert and the shell is reduced. As mentioned above, a reduced contact area increases localized stresses in shell 12.
Thus, a need exists for a means to improve the coupling between a flextensional transducer's stack driver and flexural shell. Accordingly, it is an object of the present invention to provide a means for efficiently coupling a flextensional transducer's driver to its flexural shell. Another object of the present invention is to provide a means for coupling a stack driver to a flexural shell in a flextensional transducer such that the possibility of gapping is minimized while maintaining sufficient transverse structural support for the shell over a range of shell deflections. Still another object of the present invention is to provide an acoustic transducer with means of coupling the transducer's driving means to the transducer's vibrating flexural surface such that the coupling means may be adjusted to tune the transducer's acoustic performance.